rectifier and buck-boost converter?

You have many options to charge your supercaps.  The simplest is to rectify your input voltage and run it through an LDO to set the maximum charging voltage.  Then use the supercaps as the input source for standard power supplies that convert the 2.7V into the correct regulated voltage for the uP.

Can you tell me what company is this for and and what is the application?

thanks a lot

I am not very clear about the LDO , I charge the caps directly with the rectified input without LDO , of course the charging voltage is not stable..but  it seems work...

Do you mean with the LDO , I get a stable output to charge the caps, So the cahrging efficiency would be better ?

I am from Bosch Rexroth

the application is a test of a batteryless sensor, also called "energy harvesting"

I think I understand what you want to do. You have an high impedance AC source generated by an harvester (possible inductive) and now you need to charge a super cap to provide power for the MCU.

Here we also recommend to rectify the AC source what you already do. Now you need to charge the super cap with a regulated voltage limiting the max voltage to 2.7V of the super cap or even lower due to long term reliability resosns of the super cap. When the super cap is empty the challenge is to limit the current into the super cap to avoid that your AC source drops due to its high impedance. We could recommend the TPS62120 a 15V step down converter designed for Energy Harvesters. There are two challenges you need to verify when testing the application. 1. In-rush current of the converter when the super cap is empty. TPS62120 limits the inrush current but it needs to be verified with your AC source. Secondly the stability of the converter using a large output cap like the super cap.

thanks Oliver

So you mean  the caps should be charged with a stable voltage ? and the TPS62120 will convert my rectified output to a stable output to charge the caps ? The Input Voltage Range of TPS62120 is 2V to 15V ,my rectified output is 0- almost 15V, and I could  get a stable 5V to charge the caps(two 3.3F 2.7V in serial connect) ? So the TPS62120 will act like a buck-boost converter ?  For my application, the harvester can not give a continuous output ,so I want this uncontinuous output go into the caps and then generate a  continuous stable output to power the MCU for 1 min ,then put the MCU  in sleep and the cap will be charged till it could power MCU again.  Maybe I could connect another TPS62120 after caps so I can get output to power the MCU ?

thanks a lot

The TPS62120 is a buck converter and can't be used as a buck-boost converter. Having two super caps in series requires additional balance resistors or circuitry. Please  regulate the TPS62120 directly to the voltage required by your MCU. So you need only one buck converter. Drawback is that you do not convert all the energy of your AC source since the buck converter operates only down to 2V. For a final answer we would need MCU supply voltage/current, AC source impedance or current capability, block diagram etc.

Hopefully I could guide you at least in the right direction....

thanks

to power the MCU I would use TPS63000 after the super caps,this one works as a buck-boost converter

To charge the caps I want to use as much energy  as possible from the AC source, here I am not clear what to do....